Theory Amps and Watts

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zcanyonboltz

Senior Member
Location
denver
So I've searched this online for a while and I keep getting taken to physics websites and forums and long drawn out answers that don't make a lot of sense to a non physicist. My question is how is it that when voltage increases amperage decreases but watts stay the same? To illustrate, lets say you have 10,000 watts being drawn at 120 volts, the amperage is 83.3 amps. Now take that same 10,000 watts at 240 volts and you have 41.6 amps. People often think just because equipment operates at 240 volts the power consumption is less but when I tell them this isn't the case you're billed in wattage consumption and this stays the same at either 120v or 240v they are in awe. Any ways to explain this? Thanks
 

PetrosA

Senior Member
Imagine a two handled saw that cuts in both directions. It takes a number of passes (ex. 100) to cut through a log. Now imagine two teams sawing. One team is made up of one sawyer, the other of two. The amount of work done is the same whether it's one person cutting or two people cutting but either the amount of time needed is different or the speed of the sawyer will have to be different to accomplish the task in the same amount of time.

It's not a perfect example, but may help people understand the concept.
 

templdl

Senior Member
Location
Wisconsin
So I've searched this online for a while and I keep getting taken to physics websites and forums and long drawn out answers that don't make a lot of sense to a non physicist. My question is how is it that when voltage increases amperage decreases but watts stay the same? To illustrate, lets say you have 10,000 watts being drawn at 120 volts, the amperage is 83.3 amps. Now take that same 10,000 watts at 240 volts and you have 41.6 amps. People often think just because equipment operates at 240 volts the power consumption is less but when I tell them this isn't the case you're billed in wattage consumption and this stays the same at either 120v or 240v they are in awe. Any ways to explain this? Thanks

The way that I look at it is that power in watts is the work that is Bering done. To do the same amount of work using various voltatsges the amount of current require to do that work will vary.
Raise the voltage the current will go down. Decrease the voltage the current required to do that same work will go up. Watts law, W= ExI
But, to do the the actual device doing that work must be redesigned to function at that voltage as simply increasing the voltage will not reduce the current but will increase it which will result in damaging the device. What is missing in your thoughts is ohms law the takes resistsnce into account I=E/R. The resistance must be increased as the voltage is increased.
I tried to put this in basic terms.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
To make things more complicated this concept works the exact opposite for resistant loads. For instance a 208 heater will draw less current at 208 then at 240 v. The wattage changes in this case

Here is an example of how resisitance heat works using ohms law
Lets take a baseboard heater that is rated 2000 watts at 240V. What is the wattage rating @208 volts?
Our intuition says as the voltage increases the amps go down however that is not the case with resistance heat.


R= V^2 / P
R= 240^2/2000= 28.8
Since the resistance does not change we can now use this formula


P=V^2 / R
P= 208^2/ 28.8= 1502 watts


So a 2000 watt heater at 240V will be rated 1502 watts at 208V
 

kwired

Electron manager
Location
NE Nebraska
Hydraulic or pneumatic analogies mostly work here.

Assume you have a pipeline which is like the conductor, driving a turbine, cylinder,etc (the load).

The more pressure (similar to voltage) you have the less flow you need (similar to electric current) to deliver a specific force (power) to the turbine or cylinder. The other way around is less pressure requires more volume of flow to do same work, and they will be proportional to each other just like voltage and current are in an electrical application.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
The advantage of 240v vs 120 IMO is the use of a smaller size conductor can be used. The example you stated is fine but generally not that drastic when it comes to motors. It also helps with distribution of loads-- 3 phase motors will evenly distribute the load on all three phases rather than just on one or even 2 phases.

You are correct it does not save money. The efficiency of the motor may help a bit with that
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
The advantage of 240v vs 120 IMO is the use of a smaller size conductor can be used. The example you stated is fine but generally not that drastic when it comes to motors. It also helps with distribution of loads-- 3 phase motors will evenly distribute the load on all three phases rather than just on one or even 2 phases.

You are correct it does not save money. The efficiency of the motor may help a bit with that

I think the '240 volts saves money' comes from decades ago when air conditioning was getting popular. In 1977 I moved to an apartment that had a 240 volt air conditioner that was basically a large window shaker. Back then, a 120 volt window shaker would really shoot up the electric bill and that was just for one room. The increase for using the 240 volt unit I had was barely noticeable. That would lead a person to think that 240 was cheaper than 120, when in reality it was just that the 240 volt commercial unit was far more efficient than a residential grade unit and probably cost 5 times as much to buy.

Not only was the motor more efficient, the design was, too.
 

Besoeker

Senior Member
Location
UK
So I've searched this online for a while and I keep getting taken to physics websites and forums and long drawn out answers that don't make a lot of sense to a non physicist. My question is how is it that when voltage increases amperage decreases but watts stay the same? To illustrate, lets say you have 10,000 watts being drawn at 120 volts, the amperage is 83.3 amps. Now take that same 10,000 watts at 240 volts and you have 41.6 amps. People often think just because equipment operates at 240 volts the power consumption is less but when I tell them this isn't the case you're billed in wattage consumption and this stays the same at either 120v or 240v they are in awe. Any ways to explain this? Thanks
You are billed for kWh.

That aside.............
Assume a single-phase resistive load as you have implied with the 83A
At 120V that 10,000W would be about 0.7 ohms.
At 240V it would be 0.17 ohms.
 

zcanyonboltz

Senior Member
Location
denver
Yes this is making sense, I understand now that the resistance in the device is manufactured to change with a different voltage, as voltage increases resistance must also increase in the device or the device will fry. In the two handled saw example the same amount of work gets done using less effort from each individual sawyer when there are two sawyers the sawyers are the amps. The 10,000 watts number is an example from HID lights which can run on 120v or 240v using an MLC. Lower cost on wire for sure operating at 240v.


You are billed for kWh.
That aside.............
Assume a single-phase resistive load as you have implied with the 83A
At 120V that 10,000W would be about 0.7 ohms.
At 240V it would be 0.17 ohms.


Besoeker what formula are you using to get ohms? When I take E/I I get 240v/41.6a=5.76 ohms and 120v/83a= 1.44 ohms. Billing in KwH is still being billed according to wattage consumption the KwH is just a unit of measurement for watts used.
 
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electricalist

Senior Member
Location
dallas tx
So when a light is on how does the meter know how much was unused if the meter doesnt read outgoing on the hot and the rest coming back on the neutral. Probably a dumb question but one I dont know.
 
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